(1) Technical Field
The present invention relates generally to semiconductor devices and more particularly to a device having a PFET and dual etch stop liners that provide an enhanced stress state to the PFET and methods for the manufacture of such a device.
(2) Related Art
In the manufacture of semiconductor devices, silicon nitride (Si3N4) liners may be used to induce a stress in a transistor channel to modulate carrier mobility. The stress induced is dependent upon the stress state of the silicon nitride liner itself and the relative location of the portion of the silicon channel of interest. For example, a tensile silicon nitride liner will produce the opposite stress beneath itself and the same stress in areas laterally adjacent itself. That is, a tensile silicon nitride liner will produce a compressive stress beneath itself and a tensile stress in areas laterally adjacent the silicon layer.
FIGS. 1-3 show the induction of tensile and compressive stresses in a silicon layer 130. Referring to FIG. 1, a tensile silicon nitride liner 160 has been overlaid on silicon layer 130 of device 100. Tensile silicon nitride liner 160 induces a compressive stress 162 in a portion of silicon layer 130 beneath itself while also inducing a tensile stress 164 in a laterally adjacent portion of silicon layer 130. Similarly, FIG. 2 shows a compressive silicon nitride liner 170 atop silicon layer 130 of device 100. Compressive silicon nitride liner 170 induces a tensile stress 174 in a portion of silicon layer 130 beneath itself while inducing a compressive stress 172 in a laterally adjacent portion of silicon layer 130.
Referring to FIG. 3, device 100 is shown having abutting tensile silicon nitride liner 160 and compressive silicon nitride liner 170, which results in enhanced compressive and tensile stresses beneath the liners. That is, while tensile silicon nitride liner 160 produces its own compressive stress 162 beneath itself, as shown in FIG. 1, the abutting compressive silicon nitride liner 170 also produces a compressive stress 172 beneath a portion of tensile silicon nitride liner 160, as shown in FIG. 2. Similarly, while compressive silicon nitride liner 170 produces its own tensile stress 174 beneath itself, as in FIG. 2, abutting tensile silicon nitride liner 160 also produces a tensile stress 164 beneath a portion of compressive silicon nitride liner 160. As shown in FIG. 3, the compressive stresses 162, 172 and tensile stresses 164, 174 produced by such an arrangement of silicon nitride liners are collectively greater than would result if the liners 160, 170 did not abut.
Such increases in stress can be useful, for example, in improving the function of certain components of a semiconductor device, particularly field effect transistors (FETs). FIG. 4 shows the preferred stress states of an n-channel FET (NFET) 240 and a p-channel FET (PFET) 250 along their longitudinal (lengthwise) and transverse (widthwise) axes, L and W, respectively. Each FET 240, 250 includes a source 242, 252, a gate 244, 254, and a drain 246, 256, respectively. The function of NFET 240 is improved when NFET 240 is subjected to tensile stresses T along both its longitudinal axis L and transverse axis W. The function of PFET 250, on the other hand, is improved when PFET 250 is subjected to compressive stress C in a direction parallel to its longitudinal axis L and tensile stress T in a direction parallel to its transverse axis W. Such improved function includes, for example, improved electron transport and improved hole transport.
Devices known in the art improve FET function by increasing tensile stresses along the FET's longitudinal axis. As shown in FIG. 4, however, FET function can also be improved by increasing tensile stresses along the FET's transverse axis. Accordingly, a need exists for a device having improved FET function due to increased transverse stress, as well as methods for the manufacture of such a device.